1. Field of the Invention
The present invention generally relates to an automatic steering system and, more particularly, to an automatic steering system for controlling a steering angle so that a vehicle traces a reference line provided along a road.
2. Description of the Related Art
Conventionally, a system enabling an automatic running or directional movement control of a vehicle path is known such as disclosed, for example, in Japanese Laid-Open Patent Application No. 1-253007. In this system, a plurality of magnetic markers are buried in a road along a predetermined path of a vehicle so that directional movement of the vehicle is automatically controlled while tracing the magnetic markers.
In the above-mentioned system a plurality of magnetic signals are generated along the moving path of the vehicle. The intensity of each magnetic signal is strong in the vicinity of the corresponding magnetic marker, and is decreased as the distance from the magnetic marker is increased. In the conventional system, a movement direction of a vehicle is controlled so that the intensity of the magnetic signal detected by a magnetic pickup provided on the vehicle is maximized. The vehicle sequentially traces the magnetic markers, and thus the automatic directional movement control of the vehicle is achieved.
However, in the above-mentioned conventional system in which a vehicle traces magnetic markers, many vehicles run almost the same path. In such a condition, wheel tracks or ruts may be formed on the road surface in a relatively short period of time. Accordingly, if the conventional system is used for constructing an automatic vehicle directional movement control system for many vehicles, the road surface must be frequently repaired to remove the wheel tracks. This increases maintenance cost of the road. Additionally, the maintenance construction of the road may limit availability of the road. Thus, the merit of the automatic vehicle directional movement control system cannot be sufficiently demonstrated.
Additionally, a directional movement control path in a curve may be varied within a lane so as to provide a safe control. That is, a vehicle may enter a curve from an outer side of a lane and move to an inner side in the middle of the curve and exit from an outer side of the curve (this running or movement path is referred to as out-in-out), or a vehicle may enter a curve from an inner side of a lane and keep the inner side until the vehicle exits the curve (this running or movement path is referred to as in-in-in). In order to achieve such a running or movement path in the conventional automatic vehicle directional movement control system, the magnetic markers provided in a curve must be buried along the out-in-out path or in-in-in path. In other words, in the conventional system in which each magnetic marker is buried in a predetermined position with respect to a width of a lane (for example, in the center of the lane) whether the lane is straight or curved, a vehicle can only take a single running or movement path. Thus, in the conventional system, there is a problem in that the entire width of a lane cannot be efficiently used.
Additionally, the running or movement path of a vehicle must be offset from a regular path during road construction or when a vehicle is stopped accidentally on a road. Thus, in the conventional system, there is also a problem in that a vehicle cannot cooperate with an unexpected event on the road.